[Sidenote: and mutual assistance of observation, experiment, and
speculation.]

Historically, no branch of science has followed this order of growth;
but, from the dawn of exact knowledge to the present day, observation,
experiment, and speculation have gone hand in hand; and, whenever
science has halted or strayed from the right path, it has been, either
because its votaries have been content with mere unverified or
unverifiable speculation (and this is the commonest case, because
observation and experiment are hard work, while speculation is
amusing); or it has been, because the accumulation of details of
observation has for a time excluded speculation.

[Sidenote: Recognition of these truths in recent times, and consequent
progress.]

The progress of physical science, since the revival of learning, is
largely due to the fact that men have gradually learned to lay aside
the consideration of unverifiable hypotheses; to guide observation
and experiment by verifiable hypotheses; and to consider the latter,
not as ideal truths, the real entities of an intelligible world behind
phenomena, but as a symbolical language, by the aid of which nature
can be interpreted in terms apprehensible by our intellects. And if
physical science, during the last fifty years, has attained dimensions
beyond all former precedent, and can exhibit achievements of greater
importance than any former such period can show, it is because able
men, animated by the true scientific spirit, carefully trained in the
method of science, and having at their disposal immensely improved
appliances, have devoted themselves to the enlargement of the
boundaries of natural knowledge in greater number than during any
previous half-century of the world's history.

[Sidenote: The three great achievements. Doctrines of (1) molecular
constitution of matter, (2) conservation of energy, (3) evolution.]

I have said that our epoch can produce achievements in physical
science of greater moment than any other has to show, advisedly; and
I think that there are three great products of our time which justify
the assertion. One of these is that doctrine concerning the
constitution of matter which, for want of a better name, I will call
'molecular;' the second is the doctrine of conservation of energy; the
third is the doctrine of evolution. Each of these was foreshadowed,
more or less distinctly, in former periods of the history of science;
and, so far is either from bei